1. Field of the Invention
Real time, on-line detection and quantitation of aqueous ammonia is a critical requirement for closed loop environmental life support systems. Dissolved ammonia and the ammonium cation are primarily of biogenic origin, resulting from the metabolic degradation of nitrogenous biomolecules such as amino acids, purines, and pyrimidines. The predominant hydrophilic metabolite, urea, is unstable with respect to hydrolysis and readily decomposes to ammonia and carbon dioxide. Hence, accurate and timely characterization of ammonia levels in closed loop reclamation streams is required to ensure proper water processor operation.
Conventional analytical techniques are generally unsuitable for continuous ammonia monitoring due to sample conditioning requirements, measurement instability, interferences, discontinuous aliquot sampling, and slow response times. For example, calorimetric determinations such as Nesslerization, or the phenate method require sample conditioning as well as reaction with chromogenic reagents. Ion chromatography requires the addition of a buffer and can only analyze selected aliquots whose interval depends on the time needed for separation and elution of ionic constituents. Ammonia ion selective electrodes require pH adjustment and the presence of ionic strength adjusting buffers, need frequent recalibration, suffer from slow response at low concentrations, and can become unstable due to contamination of the ammonia permeable membrane which then must be replaced. Although some of these techniques can be adapted to quasi-real time operation, the added cost and complexity makes them unattractive.
2. Description of Related Art Including Information Disclosed Under 37 C.F.R. 1.97 and 1.98
The subject invention was made as a real time, online detection and quantitation system for aqueous ammonia for use in a closed loop environmental life support system. However, it may be used in any aqueous process stream for detection and quantitation of ammonia.
The following references relate to detection and measurement of ammonia in liquids.
U.S. Pat. No. 4,700,709 to Kraig discloses an apparatus for determining the concentration of ammonium ion in fluid or tissue without adjusting the pH thereof, the apparatus comprising (a) an ammonia concentration measuring electrode for contacting the fluid or tissue and producing a first output signal related to ammonia concentration therein, (b) a hydrogen ion concentration measuring electrode for contacting the fluid or tissue and producing a second output signal related to hydrogen ion concentration therein, (c) temperature measuring means for contacting the fluid or tissue and producing a third output signal related to temperature therein, and (d) means for calculating ammonium ion concentration based upon the first, second and third output signals utilizing a disclosed equation.
U.S. Pat. No. 4,314,824 to Hansen et al. discloses a method of preparing a sample for treatment in which a continuous flow of liquid carrier receives sample portions, the method comprising: passing the carrier through a conduit in a manner such that flow of the carrier is laminar, unsegmented and continuous; introducing sample portions into the carrier; controlling dispersion of the sample portion in the carrier by varying at least one of the volume of the sample portion, the flow velocity of the carrier, or the dimensions of the conduit conducting the sample and the carrier. Also disclosed is an apparatus for practicing the method.
U.S. Pat. No. 3,718,433 to Emmet discloses a process for determining in an aqueous sample the content of nitrogen containing compounds from the group consisting of urea and tyrosine, through chemical reaction and spectral absorbency determination. The process comprises: (1) mixing the aqueous sample at a pH between 4.0 and 8.0 with a solution containing free chlorine; (2) mixing the resultant solution between a pH of 8.0 and 11.0 with a phenol solution; (3) determining the absorbency of the resultant solution substantially in the 454 .mu. and in the 375 .mu. region of the spectrum; and (4) comparing the resultant absorbency of step 3 at 454 mu with a standard urea sample, and the resultant absorbency of step 3 at 375 mu with a standard tyrosine sample.
U.S. Pat. No. 4,209,299 to Carlson discloses a method for determining the amount of volatile electrolyte present in an aqueous liquid sample, comprising: transferring volatile electrolyte from the sample into a second liquid of known electrical conductivity through a gas-permeable hydrophobic membrane that does not pass the aqueous liquid, during a predetermined time interval, and then determining the change in electrical conductivity in the second liquid resulting from such transfer. The invention also discloses an apparatus for practicing the method.
U.S. Pat. No. 5,158,868 to Bergkuist et al. discloses a method for measuring a constituent of interest of a biological fluid or the like comprising the steps of: providing a reaction chamber that contains an immobilized enzyme capable of modifying a constituent of interest; providing a measuring system; placing a first portion of a biological fluid to be analyzed in the reaction chamber and concurrently exposing a second unmodified portion of the biological fluid to the measuring system to provide a first data output; oscillating the first biological fluid portion with bidirectional flow in the reaction chamber to facilitate modification by the immobilized enzyme of the constituent of interest in the biological fluid; then exposing the first portion of the biological fluid to be analyzed to the measuring system to provide a second data output; and modifying the second data output as a function of the first data output to provide an indication of the actual amount of the constituent of interest in the biological fluid. The invention also discloses a detecting means that comprises an ion selective electrode and a reference electrode.
U.S. Pat. No. 3,765,841 to Paulson et al. discloses a method for determining the concentration of a component in a sample, wherein the sample, upon being introduced into solution with a reagent, reacts therewith at a rate indicative of the concentration. The method comprises: monitoring a characteristic of the solution or a component or product of the reaction which is proportional to the concentration; generating an output signal proportional to the time rate of change of the characteristic; measuring the value of the output signal; and inhibiting the measurement of the value of the output signal for a predetermined, fixed time interval from introduction of the sample into the reagent, the time interval being sufficient to permit thorough mixing of the sample with the reagent. Also disclosed is an apparatus for practicing the method.